Precision triangle waveform generator

ABSTRACT

A triangle waveform generator is set forth that comprises a capacitive element, a regulator, and a control circuit. The regulator is configured to charge the capacitive element in responsive to a first control signal and to discharge the capacitive element in response to a second control signal. The control circuit is responsive to a reference waveform to generate the first and second control signals. In one example, the control circuit generates the first and second control signals in response to the amplitude, frequency, phase, and symmetry of the reference waveform.

PRIORITY CLAIM

This application claims the benefit of priority from U.S. ProvisionalApplication No. 60/727,404, filed Oct. 17, 2005, which is incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to pulse width modulated powerconverters, and more specifically to a precision triangle waveformgenerator that generates a triangle wave used, for example, ininterleaved pulse width modulation amplifiers.

2. Related Art

Pulse width modulation (PWM) amplification for audio applications hasbeen used to increase efficiency by incorporating output devices thatact as switches as opposed to linear devices that must dissipate asubstantial amount of power. In PWM amplifiers, an audio input signal isconverted to a pulse width modulated waveform. To this end, an audiosignal is provided to the amplifier to modulate the width of arectangular waveform based, for example, on the amplitude of the audiosignal. The modulated waveform is used to drive one or more outputdevices as switches that are either fully saturated or off. The outputdevices, often implemented using switching power transistors, may bealigned in half-bridge pairs such that one device of the pair switches apositive voltage to the output, while the other device switches anegative voltage to the output. The switched output signals may beprovided to the input of a low-pass filter in an attempt to removeharmonic signals and sidebands that are beyond the spectrum of thedesired output waveform. The filtered analog signal is used to drive theload, such as a loudspeaker.

Triangle waveform generators are used to modulate the audio signal togenerate the pulse width modulated waveforms. Such triangle waveformgenerators may employ voltage controlled oscillators whose frequency isresponsive to a certain control voltage. The triangle wave generated bysuch a triangle waveform generator may be phase locked to a referencefrequency. The triangle wave may be modulated by the control voltage tomap the amplitude into pulse width. Presently, triangle waveformgenerators are not capable of concurrently controlling triangle one ormore of the wave amplitude, frequency, symmetry and/or phase. Thus,systems and methods to more accurately control the quality of thetriangle waveform are needed.

SUMMARY

A triangle waveform generator is set forth that comprises a capacitiveelement, a regulator, and a control circuit. The regulator is configuredto charge the capacitive element in responsive to a first control signaland to discharge the capacitive element in response to a second controlsignal. The control circuit is responsive to a reference waveform togenerate the first and second control signals. In one example, thecontrol circuit generates the first and second control signals inresponse to the amplitude, frequency, phase, and symmetry of thereference waveform.

Other systems, methods, features and advantages of the invention willbe, or will become, apparent to one with skill in the art uponexamination of the following figures and detailed description. It isintended that all such additional systems, methods, features andadvantages be included within this description, be within the scope ofthe invention, and be protected by the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. Moreover, in the figures, likereferenced numerals designate corresponding parts throughout thedifferent views.

FIG. 1 is a schematic block diagram of a pulse width modulated amplifierhaving an interleave order of two.

FIG. 2 is a schematic block diagram of an exemplary phase frequencycontrol system that may be employed in the system shown in FIG. 1.

FIG. 3 is a schematic block diagram of an exemplary triangle wavegeneration system that may be employed in the system shown in FIG. 1.

FIG. 4 is a schematic block diagram of a further exemplary phasefrequency control system that may be employed in the system shown inFIG. 1.

FIG. 5 is a schematic block diagram of a further exemplary triangle wavegeneration system that may be employed in the system shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of an example interleave of two (N=2)full-bridge pulse width modulated (PWM) amplifier 100. The interleavedPWM amplifier 100 receives an input signal from a signal source 110. Theinput signal may be split into a first branch 112 and a second branch114. The first branch 112 includes an inverting block 120 configured toinvert the input signal. The inverting block 120 is connected with afirst pulse width modulator 130, referenced as PWM A. PWM A 130 isconnected with a first half-bridge 150, referenced as half bridge A. Theoutput of half-bridge A 150 is then connected with a load 160.

The inverted input signal may be modulated onto a first trianglewaveform with N=2 PWM modulation by the PWM A 130. Generation of thefirst triangle waveform involves generation of a switching frequency(Fs) for each output channel supplied to the load 160 using a phasefrequency control system 170. The switching frequency(s) (Fs) may beused by a triangle wave generation system 180 to generate the firsttriangle waveform. Modulation of the inverted input signal onto thefirst triangle waveform results in a first control signal. The firstcontrol signal may be provided to the first half-bridge 150 to controlthe power output to the load 160. The second branch 114 includes anon-inverting block 125 that is connected with a second pulse widthmodulator 135, referenced as PWM B. PWM B 135 similarly modulates thenon-inverted input signal onto a second triangular waveform to generatea second control signal using a phase frequency control system 170 and atriangle wave generation system 180. The second control signal may beprovided to a second half-bridge 155, referenced as PWM B, to controlthe power output to the load 160.

In one example, the phase frequency control system 170 and the trianglewave generation system 180 may be functionally provided in the form ofan integrated circuit. Such an integrated circuit would allow lower costand a smaller size. Multiple channels of triangle waveform generationcould be included within one package. The integrated circuit may beoperated from a common reference clock with exacting phase control. Inother examples, the functionality of the phase frequency control system170 and the triangle wave generation system 180 may be formed asseparate components, or in any combination of separate components andone or more integrated circuits.

FIG. 2 is a block diagram of an example phase frequency control system200 for a pulse width modulator. The phase frequency control system 200may be formed as an integrated circuit and/or as interconnectedindependent components. The following discussion of an exampleconfiguration is based on a phase frequency control system 200 formed inan integrated circuit. In FIG. 2, a master oscillator 202 is operatedwith a master crystal 204 to generate a master oscillator output signalon a master output line 206 when enabled by a master slave (M/S) signal207. The master crystal 204 may be external to the integrated circuit asindicated by the circles in FIG. 2 representative of input/output pinson the integrated circuit. The master crystal 204 may dictate apredetermined frequency, such as 16 MHz, from the wide range offrequencies for which a crystal may be used to generate a frequency. Thefirst output signal 206 may be provided to an OR gate 208. A voltagecontrolled oscillator 210 may provide a voltage controlled output signalon a voltage controlled line 212 to the OR gate 208 when enabled by aslave master (S/M) signal 216. The voltage controlled oscillator 210 mayreceive a control signal from a phase frequency detector charge pump218.

The voltage controlled oscillator 210 may be a slave that operatesindependent of the master oscillator 202 when the master oscillator 202is unavailable. Alternatively, since the master oscillator 202 operateswith a fixed predetermined frequency, the master oscillator 202 may bedisabled and the voltage controlled oscillator 210 may be enabled tooperate the phase frequency control system 200 at other than a fixedpredetermined frequency.

An output of the OR gate 208 may be supplied to a first synchronouscounter 224 and a second synchronous counter 226. The first and secondsynchronous counters 224 and 226 are configured to count full length andprovide output bits that all change at the same time to provide phasecontrol between a plurality of output channels 228. In one example, thefirst and second synchronous counters 224 and 226 may be 6 bitsynchronous counters. The first and second synchronous counters 224 and226 may be operated with a phase frequency register 232 that provides afirst value for the first synchronous counter 224. Alternatively, thefirst and second synchronous counters 224 and 226 may have separate andindependent registers.

The register(s) 232 of the first and second synchronous counters 224 and226 may be non-volatile and loaded with values that result in a fixedfrequency control by the phase frequency control system 200.Alternatively, the register(s) 232 may be volatile and may be loadedwith one or more values that result in a variable frequency. Forexample, only the second synchronous counter 226 may be provided withnon-volatile memory in which a value may be stored. In this example,following power up and operation to increment the second synchronouscounter 226, the second synchronous counter 226 may load a value intothe first synchronous counter 224 when the second synchronous counter226 counts to a predetermined condition. In yet another alternative, oneor both values may be loaded into the register(s) 232 by an I2C signalon an I2C signal line 234. In other examples, any other communicationsystem and/or protocol may be used to download values into theregister(s) 232.

The output channels 228 are switching frequency signals (Fs) that aremaintained as internal signals within the integrated circuit andprovided to the triangle wave generation system 180 (FIG. 1). In FIG. 2,there are four output channels 228, in other examples, any other numberof output channels 228 may be generated depending on the load beingsupplied. A first switching frequency signal may be provided on a firstchannel (Ch1 Fs) 236. The first switching frequency signal may begenerated from the first synchronous counter 224 when a predeterminedvalue is reached on a predetermined bit, such as bit Q5, of thesynchronous counter 224.

The first switching frequency signal may be delayed by a buffer 238, andthen provided as an input to a second output channel flip-flop 240. Thesecond output channel flip-flop 240 may be toggled based on a clocksignal provided as an output value from the synchronous counter 224. Theoutput value may be another bit, such as bit Q3, of the firstsynchronous counter 224. The output of the second output channelflip-flop 240 may be a second switching frequency signal (Fs) providedon a second channel (Ch2 Fs) 242. A third output channel flip-flop 244and a fourth output channel flip-flop 246 may be further sequentiallydelayed before generating a third switching frequency signal provided ona third channel (Ch3 Fs) 248 and a fourth switching frequency signalprovided on a fourth channel (Ch4 Fs) 250, respectively. Each of thefirst, second, third and fourth switching frequency signals may beoffset from each other by determined amount so that the interleavedpower converter produces vectors that are evenly spread, such as byabout 22.5 degrees. In other examples, other systems and methods may beused to generate the offset first, second, third and fourth switchingfrequency signals on the respective channels.

The phase frequency control system 200 also includes a frequency divider260, a balanced receiver 262 and a balanced transmitter 264. Thefrequency divider 260 may be used to generate a lower frequency when thevoltage controlled oscillator 210 is enabled. The generated frequencymay be lower than the frequency of an external reference clock 266provided from another master clock, such as another integrated circuitor a power supply. The external reference clock 266 may be provided tothe frequency divider 260 by the balanced receiver 262. The externalreference clock 266 may be received from a source external to theintegrated circuit of the present example, as illustrated by the circlesdepicting input/output pins of the integrated circuit. The balancedreceiver 262 may operate as a buffer that is balanced to reduceelectromagnetic fields (EMF) and improve signal-to-noise ratios (S/N)within the integrated circuit. Accordingly, when the master oscillator202 is not available (or disabled), the voltage controlled oscillator210 and the phase frequency detector charge pump 218 may operate at thefrequency reference and/or one or more reduced frequencies provided bythe frequency divider 260 based on the frequency reference provided bythe balanced receiver 262.

When the master oscillator 202 is enabled, the balanced transmitter 264may similarly be enabled with the same master/slave signal 207. Thebalanced transmitter 264 may provide a signal representative of themaster oscillator 202 as the external reference clock 266. When themaster oscillator 202 is operating, the external reference clock 266 maybe provided as a frequency reference to other devices external to theintegrated circuit.

During operation, the phase frequency control system 200 is capable ofgenerating a plurality of channels of switching frequency (Fs) signalsthat are offset in phase from each other by a determined amount. Thepulse width modulator may modulate waveforms in the form of triangularwaveforms that encode amplitude information into pulse width informationbased on the switching frequency (Fs) signals.

The ability to control phase of the switching frequency signals (Fs) isuseful to minimize signal corruption resulting from switching noisecrosstalk. If all the output channels 236, 242, 248 and 250 are operatedin precise synchrony, there is maximum opportunity for crosstalk. Theact of phasing channels such that they have a maximum amount of timebetween each others switching events minimizes signal corruption fromcrosstalk transients. In other words, the distance between switching ofpairs of switches may be maximized to minimize crosstalk.

The phase frequency control system 200 may include precise control ofrelative phase to divide the modulation period into evenly temperedintervals between all of the output channels 236, 242, 248 and 250. Thephase frequency control system 200 also may operate to control phasewhen interleaved modulation is required at interleave numbers of four ormore. A single triangle waveform is adequate for an interleave of two asillustrated in FIG. 2.

FIG. 3 is a schematic block diagram of a triangle wave generation system300 that may be used to implement the triangle wave generation system180 illustrated in FIG. 1. The triangle wave generation system 300 maybe formed as an integrated circuit and/or as interconnected independentcomponents. The following discussion of an example configuration isbased on a triangle wave generation system 300 formed in an integratedcircuit.

In FIG. 3, the triangle wave generation system 300 includes a voltagecontrolled oscillator 301 and a phase frequency detector charge pump303. A triangle wave is formed with the voltage controlled oscillator301 by charging and discharging a capacitor Ct 302. Capacitor Ct 302 maybe of an advantageous form for an integrated circuit in that it has oneend grounded and the other end driven. This may be advantageous sincefringing capacitance is relatively large with integrated circuitcapacitors and the fringing capacitance is now in parallel with theintended capacitance.

Capacitor Ct 302 may be selectively charged and discharged with a chargeregulator 304 and a discharge regulator 306. The charge regulator 304includes a charge current switch 308 and a charge current source 309.The discharge regulator 306 includes discharge current switch 310 and adischarge current source 311. The charge current source 309 and thedischarge current source 311 may be supplied with a supply voltage (Vdd)314, such as 5 volts DC and ground. In addition, the magnitude ofcurrent supplied by each of the current sources 309 and 311 and thecurrent switches 308 and 310 may be controlled.

Selective switching of the respective current switches 308 and 310 andcontrol of the respective current sources 309 and 311 may control avoltage (Vt) 316 present on the capacitor Ct 302. The voltage (Vt) 316may be compared with a positive potential voltage Vtp 318 and a negativepotential voltage Vtn 320 by a first comparator 322 and a secondcomparator 324 included in the voltage controlled oscillator 301.

The positive and negative potential voltages Vtp 318 and Vtn 320 may berepresentative of the output rails of the PWM amplifier 100 illustratedin FIG. 1. The positive and negative potential voltages Vtp 318 and Vtn320 may be centered on a determined voltage such as a CMOS voltage of2.5 volts DC or a separate components voltage of ground potential, orzero volts. The voltage (Vt) 316 may be buffered by a buffer 328 priorto being compared by the first and second comparators 322 and 324. Thefirst and second comparators 322 and 324 may form a window detector. Thecomparator outputs may be used to toggle a hysteresis switch 329, suchas an R-S flip-flop. The hysteresis switch 329 may enable control of thecharging and discharging currents that manipulate the charge on thecapacitor Ct 302.

Control of the charging and discharging currents provided by the chargecurrent source 309 and the discharge current source 311 may be performedwith the phase frequency detector charge pump 303. The voltagecontrolled oscillator 301 may generate triangle waves when the chargecurrent source 309 and the discharge current source 311 are programmedby common frequency control signals provided by the phase frequencydetector charge pump 303. The phase frequency detector charge pump 303may control the currents of the charge current source 309 and thedischarge current source 311 to be tracking currents that areapproximately equal and opposite in magnitude.

In the triangle wave generation system 300, the charge current source309 and the discharge current source 311 are separately regulated toallow precise control of the two ramps that form the triangle waveform.A first window comparator output signal on a first window comparatoroutput line 330 and a second window comparator output signal on a secondwindow comparator output line 332 may be provided as a logic one or alogic zero by the hysteresis switch 329. The outputs of the hysteresisswitch 329 are provided to the phase frequency detection charge pump 303and are also operable to control the operation of the charge anddischarge current switches 308 and 310 in the respective first andsecond regulators 304 and 306.

The phase frequency detection charge pump 303 includes a first phasefrequency detector 340 and a second phase frequency detector 342. Duringoperation, the first and second phase frequency detectors 340 and 342each compare an edge of the triangle wave to a reference switchingfrequency signal (Fs) 344 and an inverted reference switching frequencysignal /Fs 346. The reference switching frequency signal (Fs) 344 isprovided by the phase frequency control system 200 (FIG. 2). The firstand second phase frequency detectors 340 and 342 compare the edges andthe reference signals for frequency and symmetry.

The phase frequency detectors 340 and 342 may be digital detectors thatmake the comparison and activate a group of charge pumps 350 in suchmanner as to effect the intended controls. More specifically, the groupof charge pumps 350 includes a first set of charge pumps 352 that areoperated to generate a charge current control signal on a charge currentcontrol line 354. In addition, a second set of charge pumps 356 areoperated to generate a discharge current control signal on a dischargecurrent control line 358. Thus, two separate and somewhat independentcontrol loops may independently control the charge current source 309and the discharge current source 311.

The first group of charge pumps 352 include a first main charge pump 362and a first cross-coupled charge pump 364. Similarly the second set ofcharge pumps 356 includes a second main charge pump 366 and a secondcross-coupled charge pump 368. In other examples, the number of sets ofcharge pumps and/or the number of charge pumps used may be increased ordecreased. During operation both of the first and second phase-frequencydetectors 340 and 342 may converge on zero degrees error. Under zerodegrees error conditions, both the “up” and “down” outputs of the firstand second phase-frequency detectors 340 and 342 are on, thereby turningon the charge pumps 362, 364, 356, 358 equally and oppositely, resultingin zero. Outputs on the charge and discharge current control lines 354and 358.

In one example control scheme each detector 340 and 342 may manipulatethe charge pump that controls the ramp that enabled the edge of thehysteresis switch 329 that was compared with the like referenceswitching frequency signal (Fs). With this approach, however, it is nota convergent scheme. Increasing the ramp speed of one edge will advancethe event timing of more than the intended edge. The total interactionof the edges may result in the two control loops fighting each otherduring operation. When one advances its event it advances the otherevent causing the other's controller to retard both events.

In another example control scheme as depicted in FIG. 3, to improvestability, the interaction between the two events may be a convergentsequence. When one controller increases its ramp speed the other rampspeed can be attenuated and vice-versa. Yet if one controller were toalways correct its effects with an exact counter correction of the othercontroller, then there would be limited ability of the triangle wavegeneration system 300 to gain frequency control of the voltagecontrolled oscillator 301 and lock on the reference switching frequencysignal (Fs), as the total oscillator period could be without any realcontrol.

To implement and maintain real control, filtering of control signalsthat are slow relative to the principle signals being processed andregulated may be used. Large capacitors, such as greater than 100picofarads capacitors, may be included in the triangle wave generationsystem 300 for filtering in the control circuits. Since integration oflarge capacitors into an integrated circuit may be difficult, addedpackage terminals may be included to connect the integrated circuit toexternal capacitors. Alternatively, a charge pump circuit in theintegrated circuit, as depicted in FIG. 3, may be used to filter slowercontrol signals instead of the large capacitors. Such a charge pumpcircuit may operate similar to a charge pump used with a digitalphase-frequency detector on a phase-locked or delay-locked loop. Thelock point may be at zero phase where the control signals become verynarrow and impulsive. At zero degrees the charge up and down pulses maybecome very narrow and time coincident, canceling each other's chargeoutput. With little to no ripple at lock conditions, it may berelatively easy to filter such a signal using only small capacitors in alow-leakage circuit. The ripple may be large when out of lock, but suchis not an operating mode.

The four charge pumps 362, 364, 366 and 368 may allow the implementationof a convergent controller within the triangle wave generator 301. Thecross-coupled charge pumps 364 and 368 may be weighted (Factor K) with acurrent magnitude, which is less than the main charge pumps 362 and 366.That is to say, the factor K must be less than one. If K were equal toone, frequency control may be diminished. If K were greater than one,the frequency control may be divergent. Therefore K can be some positivenumber greater than zero, but less than one. An example value of K isK=0.5.

To be stable, each control loop may have a zero introduced in itscontrol loop. Such is the purpose of a series RC network 370 include ina charge storage network of each of the first and second set of chargepumps 352 and 356. Each of the RC networks 370 includes at least oneresistor 372 and at least one capacitor 374. In other examples, otherquantities and configurations of resistors and capacitors than what isillustrated may be used. The resistance value of the resistor(s) 372 maybe relatively large, but may not require precision. Thus the resistor(s)372 may be readily integrated in a CMOS mixed-signal process.

The triangle wave generation system 300 also may include amplitudeadjustment of the triangular waveforms to enable feed-forwardcompensation of open-loop converter gain. This may be done by causingthe triangular waveforms to be proportional in amplitude to the powersupply voltages in the power converter stages, the half bridges 150 and155 of FIG. 1. When the triangular waveforms are maintained proportionalin amplitude to the power supply voltages, the gain may become constantand independent of the supply voltages. Without such compensation thegain may be directly proportional to the supply voltages.

The triangle wave generation system 300 also may operate with differentmodulating frequencies. For example a power converter used as a simpleclass-D audio amplifier may be designed with full bandwidth capabilityand can be modulating at 500 KHz whereas if the power converter weredesigned for use only at bass frequencies the converter can bemodulating at 50 KHz. The frequency and amplitude modulation demands mayact together to greatly increase the range of slopes of the trianglewave that are used. The triangle wave generation system 300 may be usedin a range of modulation from about 1 MHz to about 50 MHz due to thewide range of possible slopes achievable based on controllable variationin the triangle wave ramp speed. In another example, the triangle wavegeneration system 300 may be used in a range of modulation from about 50KHz to about 500 KHz.

The mapping of amplitude into pulse width is substantially withoutdistortion because the triangular waveforms are very linear and havesubstantially exact symmetry. That is to say, the uniform rising slopeof the triangle waveform is substantially equal in magnitude to thefalling slope. To allow an error in symmetry is to introduce unwantedPWM spectrum into the output provided to a load as a consequence ofphase modulation being added to the output PWM waveform. The trianglewave generation system 300 also may operate without extraneous noise orjitter on the waveform to avoid the likelihood of noise errors appearingin the PWM output. Such errors are magnified by the effective outputstage supply voltage.

During operation, the resulting control system may make adjustments atthe most opportune points of the triangle waveform. Such opportunepoints may be, for example, at the tips where the natural discontinuityin the triangle waveform appears at the limits of the modulationprocess. In other words, the control may grab the triangle waveform atits vertices and regulate it. The control may be formed to be stronglyconvergent in that both excess and deficient ramp speeds may becorrected proportional to their error to regulate the result.

FIG. 4 is a block diagram of another circuit that may be used toimplement the phase frequency control 170 shown in FIG. 1. Circuitssimilar to those used in FIG. 2 are shown with like referencecharacters. Like the circuit employed in FIG. 2, circuit 400 may beoperated in either of two operational modes. In a master clock mode, themaster clock signal 404 is generated based on an internal signalprovided by the master oscillator 202 in conjunction with the crystal204. In a slave mode, the master clock signal 404 is generated by VCO210 based on the output of a phase frequency detector charge pump 218that, in turn, is responsive to the external reference clock signal 266.

In the example shown in FIG. 4, the phase frequency control circuit 400generates two reference signals 406 and 408, although the circuit 400may be extended to generate additional reference signals. The referencesignals 406 and 408 in circuit 400 may have the same frequency and maybe in or out of phase with one another. Signals 406 and 408, in turn,are used as reference signals by the triangle wave generator 180 togenerate the multiple triangle waves that are used by the pulse widthmodulators 130 and 135.

Each time counter 226 reaches its terminal count, a signal is generatedat line 410 to counter 224 that directs counter 224 to load the valuestored in mode register 412 into the counter 224. In the illustratedexample, only the data stored at bits D8 through D13 of mode register412 are loaded into the counter 224. Output bits Q4 and Q5 of counter224 are connected to the inputs of a multiplexer 412. Output bit Q5 isalso provided to the input of a divider 414 that, in turn, divides thefrequency at which output bit Q5 for output at line 416. In thisexemplary architecture, the inputs to multiplexer 412 comprise a firstsignal having a frequency of MClk/32, a second signal having a frequencyof MClk/64, and a third signal having a frequency of MClk/128. Thefrequency of the clock signal provided at multiplexer output 418 to theclock input of flip-flop 420 is determined by the state of output bitsD14 and D15 of the mode register 412. Similarly, output bits D14 and D15may be used to determine the factor by which the frequency of the signalat Q5 is divided as well as the frequency of the output signal 406.Together, the phase data of bits D8 through D13 and the frequency dataof bits D14 and D15 cooperate to direct circuit 400 to generate thereference signals 406 and 408 at the desired frequency and relativephase.

FIG. 5 is a schematic block diagram of an alternative version of atriangle wave generation system 500 that may be used to implement thetriangle wave generation system 180 illustrated in FIG. 1. System 500 issimilar in many respects to the system 300 shown in FIG. 3 and,therefore, like reference characters are used.

Unlike system 300, the capacitor 505 is not reference to real ground.Rather, capacitor 505 is connected to a negative terminal of anoperational amplifier 520 so that it is charged with reference to avirtual ground voltage having a voltage level Vrsrc. This architecturefacilitates accurate charging and discharging of the capacitor 505 byreducing implementation problems within regulators 304 and 306.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible within the scope of theinvention. Accordingly, the invention is not to be restricted except inlight of the attached claims and their equivalents.

1-10. (canceled)
 11. A waveform generator comprising: a referencewaveform generator providing first and second reference signals; atriangle waveform generator generating a first triangle waveform signalby tracking the frequency and phase of the first reference signal, andgenerating a second triangle waveform signal by tracking the frequencyand phase of the second reference signal.
 12. The waveform generator ofclaim 11, where the reference waveform generator is operable in a mastermode in which the first and second reference signals are generated inresponse to a fixed clock signal generated internal to the referencewaveform generator and a slave mode in which the first and secondreference signals are generated in response to an externally generatedclock signal.
 13. The waveform generator of claim 11, where thereference waveform generator comprises: a phase/frequency registeradapted to receive data corresponding to the frequency and phase of eachof the first and second reference signals; and a counter responsive to aclock signal and data in the phase/frequency register to generate one ormore control signals used in the generation of the first and secondreference signals.
 14. The waveform generator of claim 11, where thetriangle waveform generator comprises: a first capacitive element; afirst regulator configured to charge and discharge the first capacitiveelement, where the first regulator is responsive to a first controlsignal to charge the first capacitive element and a second controlsignal to discharge the first capacitive element; a first controlcircuit responsive to the first reference signal to generate the firstand second control signals; a second capacitive element; a secondregulator configured to charge and discharge the second capacitiveelement, where the second regulator is responsive to a third controlsignal to charge the second capacitive element and a fourth controlsignal to discharge the second capacitive element; and a second controlcircuit responsive to the second reference signal to generate the firstand second control signals.
 15. The waveform generator of claim 14,where the first control circuit generates the first and second controlsignals in response to the frequency and phase of the first referencesignal.
 16. The waveform generator of claim 15, where the first controlcircuit comprises: a first set of charge pumps adapted to generate thefirst control signal; and a second set of charge pumps adapted togenerate the second control signal, where the first and second set ofcharge pumps are cross-coupled with one another to generate the firstand second control signals.
 17. The waveform generator of claim 15,where the first control circuit comprises: a window detection circuitgenerating a window comparator signal in response to the first trianglewaveform; a first phase frequency detector generating an output signalin response to the window comparator signal and the first referencewaveform; and a second phase frequency detector generating an outputsignal in response to an inverted version of the window comparatorsignal and an inverted version of the first reference waveform.
 18. Thewaveform generator of claim 16, where the first control circuit furthercomprises: a first set of charge pumps adapted to generate the firstcontrol signal; and a second set of charge pumps adapted to generate thesecond control signal.
 19. The triangle waveform generator of claim 18,where the first and second sets of charge pumps are cross-coupled withone another to generate the first and second control signals.
 20. Thewaveform generator of claim 19, where the first set of charge pumpscomprises: a main charge pump responsive to the output signal of thefirst phase frequency detector and an inverted version of the outputsignal of the first phase frequency detector, where the main charge pumpprovides a switched current Ir for use in generating the first controlsignal; and a secondary charge pump responsive to the output signal ofthe second phase frequency detector and an inverted version of theoutput signal of the second phase frequency detector, where thesecondary charge pump provides a switched current K*Ir for use ingenerating the first control signal.
 21. The triangle waveform generatorof claim 20, where the second set of charge pumps comprises: a furthermain charge pump responsive to the output signal of the second phasefrequency detector and an inverted version of the output signal of thesecond phase frequency detector, where the further main charge pumpprovides a switched current Ir for use in generating the second controlsignal; and a further secondary charge pump responsive to the outputsignal of the first phase frequency detector and an inverted version ofthe output signal of the first phase frequency detector, where thesecondary charge pump provides a switched current K*Ir for use ingenerating the second control signal.
 22. The waveform generator ofclaim 17, where the window detection circuit comprises: a firstcomparator disposed to compare the first triangle waveform with an upperthreshold voltage to generate a corresponding output signal; a secondcomparator disposed to compare the first triangle waveform with a lowerthreshold voltage to generate a corresponding output signal; and aflip-flop responsive to the output signals of the first and secondcomparators to generate the window comparator signal.
 23. The waveformgenerator of claim 22, where the flip-flop is further responsive to theoutput signals of the first and second comparators to generate theinverted version of the window comparator signal.
 24. An amplifiercomprising: a power stage; a reference waveform generator providingfirst and second reference signals having different phases; a trianglewaveform generator generating a first triangle waveform signal bytracking the amplitude, frequency, and phase of the first referencesignal, and generating a second triangle waveform signal by tracking theamplitude, frequency, and phase of the second reference signal; aninterleaved PWM amplifier generating interleaved PWM pulses in responseto an input signal and the first and second triangle waveform signals todrive the power stage. 25-34. (canceled)